1. Field of the Invention
The present invention relates to an exposure apparatus for forming a photoresist pattern on a semiconductor substrate. More particularly, the present invention relates to an optical system of an exposure apparatus for forming a photoresist pattern on a silicon wafer used as a semiconductor substrate.
2. Description of the Related Art
Semiconductor devices may be manufactured by a process including a fabrication process performed for forming electronic circuits on a silicon wafer used as a semiconductor substrate, an electrical die sorting (EDS) process performed for inspecting electrical characteristics of the semiconductor devices on the semiconductor substrate, and a packaging process performed for packaging the semiconductor devices in epoxy resins and individuating the semiconductor devices.
The fabrication process may include various unit processes such as a deposition process, a chemical mechanical polishing (CMP) process, a photolithography process, an etching process, an ion implantation process, a cleaning process, an inspection process, or other similar processes. The unit processes may be repeatedly performed to form the semiconductor devices of the semiconductor substrate.
The photolithography process may be performed to form the photoresist pattern on a layer formed on the semiconductor substrate to form the layer into a specific pattern having electrical characteristics. The photoresist pattern is used as a mask in an etching process for forming the specific pattern.
The photolithography process may include a photoresist coating process for coating the semiconductor substrate with a photoresist composition, a soft baking process for forming the coated photoresist composition on the wafer into a photoresist layer, an exposure process and a development process for forming the photoresist layer into a photoresist pattern using a photo mask or a reticle, a hard baking process for hardening the photoresist pattern on the wafer, or the like.
Patterns on semiconductor substrates have gradually decreased in size, having finer features, as an integration degree of semiconductor devices has increased. Accordingly, a resolution and a depth of focus (DOF) in the photolithography process have become more important.
The resolution and the DOF may be influenced by a wavelength of a light beam and a numerical aperture (NA) of a projection lens used in the photolithography process. Examples of the light beam used in the photolithography process may include a g-line light beam having a wavelength of 436 nm, an i-line light beam having a wavelength of 365 nm, a KrF laser beam having a wavelength of 248 nm, an ArF laser beam having a wavelength of 198 nm, a F2 laser beam having a wavelength of 157 nm, or the like.
Methods for preventing distortion of the photoresist pattern, for example, due to scattering and diffraction of the light beam transmitted through the photo mask in accordance with shrinkage of the patterns, may include using a phase shift mask (PSM) or an optical proximity correction (OPC) method.
Where a numerical aperture of the projection lens is increased to improve the resolution the DOF may deteriorate. An off-axis illumination (OAI) may be used to improve the DOF by projecting a zero-order diffracted light beam and positive first-order diffracted light beam occurring from the photo mask onto the semiconductor substrate.
Examples of the OAI may include an annular illumination (AI), a dipole illumination, quadrupole illumination, or the like. For example, U.S. Pat. No. 5,447,810 to Chen et al. discloses masks for improved lithographic patterning for off-axis illumination lithography, and U.S. Pat. No. 6,388,736 to Smith et al. discloses an imaging method using phase boundary masking with modified illumination.
FIG. 1 is a cross-sectional view illustrating an example of a light beam used in the annular illumination, and FIGS. 2 and 3 are plan views showing photoresist patterns formed using the light beam as shown in FIG. 1. FIG. 4 is a cross-sectional view illustrating another example of the light beam used in the annular illumination, and FIGS. 5 and 6 are plan views showing photoresist patterns formed using the light beam as shown in FIG. 4.
Referring to FIGS. 1 to 6, a ratio of an inside diameter and an outside diameter of an annular illumination beam 10a in FIG. 1 to a diameter of an equivalent circle 12 of a projection lens is about 0.65:0.85:1. A ratio of an inside diameter and an outside diameter of an annular illumination beam 10b in FIG. 4 to a diameter of an equivalent circle 12 of a projection lens is about 0.58:0.88:1.
Photoresist patterns 22a, 22b, 24a and 24b correspond to contact pads to be formed on a semiconductor substrate 20 in FIGS. 2, 3, 5 and 6.
FIGS. 2 and 3 show the photoresist pattern 22a and 22b formed on the semiconductor substrate 20 using the annular illumination beam 10a as shown in FIG. 1. FIG. 2 shows the photoresist pattern 22a formed on the semiconductor substrate 20 when a light beam transmitted through a photo mask is focused onto a surface of a photoresist layer on the semiconductor substrate 20. FIG. 3 shows the photoresist pattern 22b formed on the semiconductor substrate 20 when a light beam transmitted through the photo mask is focused at a point spaced by about 0.3 μm upwardly apart from the surface of the photoresist layer on the semiconductor substrate 20.
FIGS. 5 and 6 show the photoresist patterns 24a and 24b formed on the semiconductor substrate 20 using the annular illumination beam 10b as shown in FIG. 4. FIG. 5 shows the photoresist pattern 24a formed on the semiconductor substrate 20 when a light beam transmitted through the photo mask is focused onto the surface of the photoresist layer on the semiconductor substrate 20. FIG. 6 shows the photoresist pattern 24b formed on the semiconductor substrate 20 when a light beam transmitted through the photo mask is focused at a point spaced by about 0.3 μm upwardly apart from the surface of the photoresist layer on the semiconductor substrate 20.
As shown in figures, the resolution of the photoresist pattern and the DOF of the light beam may be determined in accordance with a cross-sectional shape of the light beam and a distance between the projection and the semiconductor substrate. Thus, the OAI may not cope effectively with complex and/or fine photoresist patterns, and a process margin in the photolithography process may not be sufficiently secured. Consequently, there is a need for improving the resolution and the DOF in a photolithographic process.